1. Field of the Invention
This invention relates to a self-plugging blind rivet comprising an elongate, generally cylindrical, stem having a radially enlarged head at one of its ends, and a tubular body having a shank, and a radially enlarged preformed head at one end of the shank for engaging one face of a workpiece, the stem extending through an axial bore in the body and projecting from the head end of the body, and the head of the stem abutting a radially extending face of the shank at a position spaced from the preformed head, whereby, on pulling the projecting stem and supporting the pre-formed head of the body in order to set the rivet, a portion of the shank between the preformed head and the head of the stem may be caused to collapse axially into an annular bulge or fold which projects radially outwardly of the shank, and constitutes a "blind head" which can be forced into engagement with the opposite, or "blind", face of the workpiece, and thus members of the workpiece can be fastened together between the pre-formed head and the blind head constituted by the annular bulge or fold.
2. Description of the Related Art
Usually, although not necessarily, the stem is provided with a breakneck at which it will break, when the setting of the rivet is complete, leaving a part of the stem, including the stem head, plugging the bore of the body, while another part of the stem, known as the stem-tail, is removed from the set rivet and discarded. It is usually required that rivets of the kind referred to should have the ability to clamp the members of the workpiece tightly together between the pre-formed head and the blind head, and also that the stem, or part thereof, which remains in the body of the set rivet should be retained within the body so as to reduce the risk of it becoming separated accidentally to form unwanted debris and leaving an opening through the bore.
One particularly successful and well known type of rivet of the foregoing kind, is sold under the Registered Trade Mark "AVEX", and is described in German Offenlegungsschrift No.2625023.
In the "AVEX" rivet, the head of the stem is external to the body, and abuts the end of the body remote from the pre-formed head (that is, the "tail-end" of the body). The stem has a breakneck spaced from its head and a short portion, immediately adjacent to its head, in which the diameter of the stem is reduced. A portion of the shank of the body adjacent to the tail end is crimped on to the reduced diameter portion of the stem, and has the same external diameter as the head of the stem. Due to this construction, the head of the stem remains in abutment with the tail end of the body, and has no tendency to enter the bore, during setting of the rivet, and the part of the stem which remains as a plug in the body of the set rivet is positively retained by the crimped engagement of the tail portion of the body with the reduced diameter portion of the stem.
Furthermore, it has also been found that, in the known "AVEX" rivet, although the blind head formed by bulbing of the shank of the body adequately engages the workpiece around the edge of the hole in which the rivet is installed, it does not normally enter into clamping engagement with a significant area of the blind face of the work.
With rivets of the kind referred to, we have also encountered a problem in that, occasionally the head of the stem would break off from the shank, either prematurely so that proper installation of the rivet could not be completed, or near the final stage of setting the rivet so that the broken-off stem head might form dangerous debris, and leave the fastener in an unstable condition.
We therefore sought to overcome this problem by reducing the diameter of the head of the stem and enclosing it in a counterbored portion at the end of the body remote from the pre-formed head. However, it was found that, due to the consequently reduced area of abutment between the stem head and the body, the stem head had a tendency to enter and pass along the bore of the body before it had applied sufficient axially compressive force to cause the body to collapse and form a satisfactory blind head. Since the size of the rivet is related to the diameter of the shank of the body and particularly its tail portion, it is not possible to significantly increase the area of contact between the stem head and the body without sacrificing other important parameters.
We have now found a way in which such a rivet can be made so as to perform satisfactorily even though the stem head is of sufficiently small diameter as to be enclosed by the body.